Memories are funny, aren’t they? You can be going along, minding your business, and then, all of a sudden, a memory pops into your head. Sometimes memories are triggered (触发) by a song that reminds you of the setting where you first heard it. Or it may be a smell that makes you think of a delicious meal with your friends or family. Perhaps hearing kids arguing makes you remember an unhappy situation with childhood friends. These spontaneous (自发的) memories can be “sweet,” or they might be “sour.” Did you ever wonder what’s going on in your brain that makes your memories produce positive or negative emotions?

Researchers at the Salk Institute for Biologica Studies in California have made a discovery that could help to uncover this secret. The explanation is a bit technical (for those of us who are not brain scientists), but basically it all focuses on a single molecule (分子) called neurotensin (神经降压素). Our surrounding environment is constantly presenting our brain with stimuli (刺激). The molecule neurotensin acts as a key indicator of whether an experience is labeled as positive or negative, depending on the nature of the stimuli. Our brain subsequently associates similar stimuli with the pleasant or painful memory that we experienced.

The study by the Salk Institute was published in the journal Nature. The focus of their experiments was the brain circuitry of laboratory mice. After a sound was played, the mice received either a pleasant swallow of sugary water or, an unpleasant electrical shock or puff of air. The researchers discovered that neurotensin was the signal that associated the sound with reward or with punishment When the gene (基因) that regulates neurotensin was removed from the mice’s brains, they had more difficulty connecting the sound with the sugary water. However, the negative responses were enhanced.

Understanding how neurons in the brain respond to neurotensin could have influences on one’s mental health. Addictions may be caused by overly high levels of neurotensin (connected with positive rewards). On the other hand, depression and anxiety can result when decreased levels of neurotensin allow negative responses to dominate. Thus, regulating neurotensin could help to maintain mental stability. One other thing: The research study showed that the brain may tend to be a pessimist, focusing on fearful memories that cause negative emotional responses.

Perhaps insights into neurotensin will help us have fewer sour memories and more sweet ones.

To call someone bird-brained in English means you think that person is silly or stupid.

But will this description soon disappear from use in the recent research? It seems English may have been unfair in association bird’s brains with stupidity.

In an attempt to find out how different creatures see the world，psychologists at Brown University in the USA have been comparing the       behaviour of birds and humans.One experiment has involved teaching pigeons to recognize letters of the English alphabet.The birds study in “classrooms”，which are boxes equipped with a computer.After about four days of studying a particular letter, the pigeon has to pick out that letter from several displayed on the computer screen.Three male pigeons have learned to distinguish all twenty-six letters of the alphabet in this way.

A computer record of the birds7 four-month study period has shown surprising similarities between the pigeons’ and human performance.Pigeons and people find the same letters easy, or hard，to tell apart.For example, 92 percent of the time the pigeons could tell the letter D from the letter Z.But when faced with U and V ( often confused by English children)，the pigeons were right only 34 percent of the time.

The results of the experiments so far have led psychologists (心理学家）to conclude that pigeons and humans observe things in similar ways.This suggests that there is something basic about the recognition process.If scientists could only discover just what this recognition process is, it could be very useful for computer designers.The disadvantage of a present computer is that it can only do what a human being has programmed it to do and the programmer must give the computer precise，logical instructions.Maybe in the future，though，computers will be able to think like human beings.

Social distancing is not a new concept in the natural world, where infectious (传染) diseases are commonplace. Through specialized senses animals can discover certain diseases and change their behavior to avoid getting ill.

In 1966, while studying chimps (猩猩) in a Tanzanian national park, zoologist Jane Goodall observed a chimp named McGregor who had caught a highly infectious virus. His fellow chimps attacked him and threw him out of the group. In one instance, McGregor got close to chimps in a tree. He reached out a hand in greeting, but the others moved away without a backward glance.

“For a full two minutes, old McGregor sat still, staring after them,” Goodall notes in her 1971 book In the Shadow of Man. “It’s really not that different to how some societies react today to such a misfortune.”

Not all animals are so aggressive toward their sick neighbors. Sometimes it’s as simple as avoiding those who may infect you.

When Kiesecker, a lead scientist in America, studied American bullfrog in the late 1990s, he found that bullfrogs could not only discover a deadly smell of infection in other bullfrogs, but healthy members actively avoided those that were sick. Bullfrogs depend on chemicals signals to determine who is sick or not.

Caribbean lobsters also shun diseased members of their community, well before they become infectious. It takes about eight weeks for lobsters infected with the deadly virus Panulirus argus mininuceovirus to become dangerous to others. Normally social animals, lobsters begin keeping away from the diseased as early as four weeks after infection - once the lobsters can smell certain chemicals released by sick individuals.

Overall, it’s important to note that, unlike us, animals don’t realize if they stay home, they might actually reduce the infection rate,” Kiesecker explains. “As humans, we have that ability. It’s a big difference.”